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Application Of Pearl Pigment On Coating System
Jun 11, 2018

While powder coating is the fastest growing finishing process in the coatings industry, it is important to understand some of the current limitations. This section outlines the most crucial technical difficulties associated with using effect pigments in powder coating systems so that powder coaters can be prepared to address them. Our researchers are also developing products that will further improve performance of applications in the powder coating system.

Currently, there are two types of application systems: corona and tribo-charging. Both require effect pigments to be bonded for the application process to work most effectively in a powder-coating system. The corona application process is used most often. Tribo-charging generally is used on a very limited basis and reserved for complicated geometric-shaped parts.

In corona applications, in order for the powder coating to transfer to the grounded part, it must first pass through an electrostatic field where it accepts a negative charge and then adheres to the positively grounded part. Although the powder coating's dryblend mixture appears uniform, mica-based effect pigments have inert properties that affect their ability to accept a charge. This parameter is critical to successful powder coating applications.

Because mica-based effect pigments do not readily carry a charge, they can easily separate from the rest of the "charged" coating during their path to the part. The difference in chargeability results in a decrease in the transfer efficiency of the pearl pigment. In other words, during spray-out a portion of the uncharged pearl separates from the powder on the way to the part and falls into the collection system, which could lead to multiple problems. The most obvious result would be an uneven or mottled coating. Uncharged pearl could also collect on the deflector tip, accumulate to a large clump and then spit onto the panel. If this occurred, the concentration of the pearlescent pigment on the panel would fluctuate continuously, making it impossible to produce a consistent finish on the part in day-to-day operations.

Additionally, if the composition of the powder fluctuates in the application process the oversprayed powder cannot be recycled, since the pigment concentration is different from that of the virgin powder. Instead, coaters must spray to waste, which greatly reduces the economic benefit of using a powder coating. However, effect pigments that are bonded act as the resin does, which reduces the aforementioned shortcomings and will be recyclable or reusable.

In the corona process, it is important to monitor the charge or kilovolt (kV) read-out on the control unit. In most situations, the voltage should be no greater than 80kV for optimum spray conditions. Higher voltages could cause a problem called back ionization. This condition occurs during electrostatic application of powder where an excessive build-up of charged powder particles limits the amount of powder that can be deposited on the substrate. The additional powder sprayed to the panel is repelled from the part, taking with it previously coated powder and forming craters.

Researchers continue to refine the efficiency of today's application methods. As an example, the powder slurries used for automotive clearcoat applications appear to be promising. In this system, the powder resin is dispersed in water. The resulting finish, which is achieved by increasing flow and leveling characteristics of the powder system, resembles the smoothness of a liquid coating.

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